The invention relates to a method for producing electrical cables coated with cross-linked polyethylene
Such a method is known from CH 618 450 A1. In this so-called silane cross-linking, reactive low-molecular compounds are grafted on a polymer radical produced by peroxides resulting in polyfunctional coupling points on the macromolecule.
These side branchesxe2x80x94e.g., organosilanesxe2x80x94are capable of reacting with other cross-links, so that theoretically a plurality of macromolecules are interconnected at such cross-linking nodes. Grafting is effected by mechanically stressing the components, e.g., by intensive mixing and kneading in a mixer or extruder at elevated temperatures.
The actual cross-linking occurs in a second step in the presence of a catalyst under the influence of water. The catalyst is required to achieve suitable cross-linking rates. For this purpose, the electrical cables provided with the insulating coating are placed in a high-humidity atmosphere or in a water bath. Since the water can get into the polymer only by diffusion, both the temperature and the wall thickness of the product to be cross-linked affect the cross-linking time.
In cable technologyxe2x80x94particularly in the production of medium voltage cablesxe2x80x94the so-called Monosil method and the Sioplas method have become known. The Monosil method is a single-step method, i.e., the individual components are placed into an extruder. Mixing, homogenizing, grafting and extruding are thus accomplished in a single working step.
In the Sioplas method, the components, except for the catalyst, are placed into an extruder, where they are mixed, homogenized, grafted and subsequently granulated. The granulate together with a separately packed catalyst batch is then usually supplied to the cable factory. At the factory, the two componentsxe2x80x94grafted granulate and catalyst batchxe2x80x94are placed into an extruder, melted, extruded onto the electrical cable and then cross-linked in the presence of water or steam.
Medium or high-voltage cables with a polyethylene insulation coating may have defects in the form of voids, impurities as well as surface defects that could lead to so-called electrical tree formation. In the presence of moisture and electrical stress, water trees may occur, which eventually destroy the insulation coating.
It has been attempted to prevent these disadvantages by using triple extrusion, i.e., by simultaneously producing an inner conductive layer, an insulation layer and an outer conductive layer through coextrusion by means of a triple extruder head. This makes it possible to minimize impurities and voids in the insulation layer or between the layers.
It has also been attempted to optimize the insulation material. A method has been described in which water tree inhibitors derived from polymers, e.g., organopolysiloxanes, polyethylene glycols, epoxy resins, polypropylene, polystyrene, and polyvinyl alcohol, are added to the polyethylene during its production. The desired effects and the influence on water tree growth vary widely. In many instances, the additives are detrimental to the homogeneity and the resistance to aging of the polyethylene as well as the electrical properties of the polyethylene.
The above optimizations have not yet been tried with silane cross-linking, so that no predictions can be made as to the influence of these additives on the cross-linking mechanism in silane cross-linking.
Thus, an object of the present invention is to provide a method for producing medium or high-voltage cables by means of silane cross-linking to provide a cost-effective cable with increased resistance against water treeing and consequently a longer service life.
This object is attained by a method for producing electrical cables coated with cross-linked polyethylene, in which a polyethylene granulate is mixed with a liquid silane-containing cross-linking agent, the granulate mixture thus prepared is melted in an extruder and extruded onto the electrical cable, and the extruded coating is cross-linked in the presence of water or steam, wherein the granulate mixture comprises a mixture of a granular material of a polyethylene homopolymer and a copolymer of ethylene, and wherein the copolymer content in the insulating coating on the cable is between 1 and 8% by weight. In this method, the granulate mixture can be coated with a liquid mixture of silane, peroxide and possibly a stabilizer prior to a compounding process, or it can be coated with a liquid mixture of silane, peroxide and possibly a stabilizer during the compounding process. Also, in this method, the granulate material coated with cross-linking agent can be grafted, homogenized and subsequently regranulated; the regranulate provided with a catalyst or a catalyst batch can be introduced into an extruder, extruded onto the electrical cable, and the coating extruded onto the electrical cable is cross-linked in the presence of water or steam, or the granulate polyethylene homopolymer material alone can be coated with the liquid cross-linking agent in a compounding system, melted, grafted, homogenized and subsequently regranulated, and the regranulate and a granular copolymer of ethylene and a catalyst, are placed into an extruder, where the mixture is melted, homogenized and extruded onto the electrical cable, wherein the compounding system can include an extruder. In the method, the copolymer of ethylene used can be an ethylene-acrylate copolymer which is an ethylene butyl acrylate (EBA), an ethylene ethyl acrylate (EEA) or an ethylene methyl acrylate (EMA), and the acrylate content in the copolymer of ethylene is 10%-35% by weight. In the method, a granular material of polyethylene homopolymer and copolymer of ethylene can be placed into an extruder, a liquid mixture of silane, peroxide and possibly a stabilizer as well as a catalyst or a highly concentrated catalyst batch is likewise placed into the extruder, and the mixture is melted, grafted and homogenized in the extruder, and the grafted, homogenized material is extruded onto the electrical cable and cross-linked in the presence of water or steam.
A significant advantage of the invention is that the use of a mixture of a polyethylene homopolymer and polyethylene copolymer with a limited copolymer content can significantly increase the resistance of the cable insulation against the formation of water trees. An LDPE and/or LLDPE-based polyethylene homopolymer may be used, which is offered in bulk at low cost by many polymer manufacturers primarily for processing into polyethylene foils. This eliminates costly xe2x80x9cspecialtiesxe2x80x9d offered by polymer manufacturers.
Further features and advantages of the invention will be apparent from the detailed description below and the appended claims.